ULTRASOUND ARTIFACTS AND CONTRAST
BY DR.ARJUN SOMI REDDY
ULTRASOUND ARTIFACTS
Types of artifacts
• Artifacts associated with ultrasound beam characteristics.
• Artifacts associated with multiple echoes.
• Artifacts associated with velocity errors.
• Artifacts associated with attenuation errors.
ARTIFACTS ASSOCIATED WITH ULTRASOUND BEAM CHARACTERISTICS.
• ultrasound beam exits the transducer as a complex three-dimensional bow-tie shape with additional off-axis low-energy beams – side lobes.
• A strong reflector located outside of the main ultrasound beam may generate echoes that are detectable by the transducer. These echoes will be falsely displayed as having originated from within the main beam.
BEAM WIDTH ARTIFACT• Caused due to the widening of the main beam after the
focal spot.
• Image quality may be improved by adjusting the focal zone to the level of interest and by placing the transducer at the center of the object of interest.
LONGITUDINAL SCAN OF THE BLADDER
BEAM WIDTH ARTIFACT
SIDE LOBE ARTIFACTS• Side lobes are multiple beams of low-amplitude
ultrasound energy that project radially from the main beam axis, mainly seen in linear-array transducers.
SIDE LOBE ARTIFACT
ARTIFACTS ASSOCIATED WITH MULTIPLE ECHOES
REVERBERATION ARTIFACTS
• US assumes that an echo returns to the transducer after a single reflection and that the depth of an object is related to the time for this round trip.
• In the presence of two parallel highly reflective surfaces, the echoes generated from the ultrasound beam may be repeatedly reflected back and forth before returning to the transducer for detection.
• The echo that returns to the transducer after a single reflection will be displayed in the proper location. The sequential echoes will take longer to return to the transducer, and the ultrasound processor will erroneously place the delayed echoes at an increased distance from the transducer.
• At imaging, this is seen as multiple equidistantly spaced linear reflections and is referred to as reverberation artifact.
COMET TAIL ARTIFACT
• Comet tail artifact is a form of reverberation.
• In this artifact, the two reflective interfaces and thus sequential echoes are closely spaced. On the display, the sequential echoes may be so close together that individual signals are not perceivable.
• In addition, the later echoes may have decreased amplitude secondary to attenuation; this decreased amplitude is displayed as decreased width.
• The result is an artifact caused by the principle of reverberation but with a triangular, tapered shape.
RING DOWN ARTIFACT
• In ring-down artifact, the transmitted ultrasound energy causes resonant vibrations within fluid trapped between a tetrahedron of air bubbles. These vibrations create a continuous sound wave that is transmitted back to the transducer. This phenomenon is displayed as a line or series of parallel bands extending posterior to a gas collection.
The display shows a bright reflector with an echogenic line extending
posteriorly.
Left lateral decubitus US image of the gallbladder shows air and fluid in the
duodenum causing ring-down artifact.
MIRROR IMAGE ARTIFACTS
• Mirror image artifacts are also generated by the false assumption that an echo returns to the transducer after a single reflection.
• In this artifact, the primary beam encounters a highly reflective interface. The reflected echoes then encounter the “back side” of a structure and are reflected back toward the reflective interface before being reflected to the transducer for detection.
ARTIFACTS ASSOCIATED WITH VELOCITY ERRORS
SPEED DISPLACEMENT ARTIFACT
• When sound travels through material with a velocity significantly slower than the assumed 1540 m/sec, the returning echo will take longer to return to the transducer.
• The image processor assumes that the length of time for a single round trip of an echo is related only to the distance traveled by the echo.
• The echoes are thus displayed deeper on the image than they really are.
• This is referred to as the speed displacement artifact
This artifact is encountered when the ultrasound beam encounters an area of focal fat.
REFRACTION ARTIFACTS
• A change in velocity of the ultrasound beam as it travels through two adjacent tissues with different density and elastic properties may produce a refraction artifact.
• In refraction, non-perpendicular incident ultrasound energy encounters an interface between two materials with different speeds of sound. When this occurs, the incident ultrasound beam changes direction.
• The ultrasound display assumes that the beam travels in a straight line and thus misplaces the returning echoes to the side of their true location.
ARTIFACTS ASSOCIATED WITH ATTENUATION ERRORS
• When the ultrasound beam encounters a focal material that attenuates the sound to a greateror lesser extent than in the surrounding tissue, the strength of the beam distal to this structure will be either weaker or stronger than in the surrounding field.
• Attenuation is also dependent on the frequency of the ultrasound. Attenuation increases with increase in frequency.
• In soft tissues, the relationship between attenuation and frequency is linear. In bone and water, attenuation increases as the square of the frequency. In clinical imaging, the different tissues an ultrasound beam encounters attenuate the beam differently.
COLOR COMET-TAIL ARTIFACT
• The color comet-tail artifact is a rapidly alternating color Doppler signal that occurs immediately deep in relation to the object causing it. The artifact appears on images as a linear aliased band of color.
• Can be accentuated by using maximum color gain and low frequency curvilinear probes.
• Also called ‘twinkling sign’.
• This artifact is not always reproducible, the causes not being evident.
• Can be used to image calcifications in various organs where grey scale ultrasound may not be helpful.
USES
• Is critical in identifying stones in the CBD especially in mildly dilated or normal ducts.
• The artifact is helpful in imaging of patients with cystic fibrosis in whom intrahepatic biliary stones are suspected clinically but there are few or no gray-scale findings.
• Is useful in diagnosing subtle pancreatic and splenic calcifications.
• In the kidney it is useful in diagnosing small calculi and early nephrocalcinosis.
• There is higher sensitivity for identifying calculi in the ureter, PUJ and cyst calcifications when this artifact is looked for.
• The color comet-tail artifact is helpful in evaluating intimal plaques in the carotid system, abdominal aorta, and peripheral arterial system.
• The color comet-tail artifact has been used in identification of catheter tips and surgical clips, can aid localization of needle tips during sonographically guided biopsy, and can even help identify foreign bodies. In appendicitis, an appendicolith can produce the artifact.
7 MHz 1.75 MHz
COLOR DOPPLER SONOGRAM SHOWS PROMINENT COLOR COMET-TAIL ARTIFACT FROM STONE (ARROW) WITHIN NONDILATED COMMON BILE DUCT.
COLOR COMET-TAIL ARTIFACT CAN HELP DIFFERENTIATE POLYPS ON NONDEPENDENT WALL OF GALLBLADDER FROM ADENOMYOMATOSIS.
CONTRAST ENHANCED ULTRASOUND (CEUS)
ULTRASOUND CONTRAST
• Composed of microbubbles.
• Consists of a microbubble shell usually made of albumin, galactose, lipid or polymers and a gas core composed of air or heavy gases like perfluorocarbon or nitrogen.
• Microbubbles have a high degree of echogenicity. The echogenicity difference between the gas in the microbubbles and the soft tissue surroundings of the body is immense. Thus the microbubble contrast agents enhances the reflection of the ultrasound waves, to produce a unique sonogram with increased contrast due to the high echogenicity difference.
• CEUS requires contrast-specific software on the ultrasound equipment that suppresses the signal from the background tissue leaving only the signal from the microbubbles.
• Pulse inversion harmonic imaging is used whereby two signals are sent down a single scan line and the second is a mirror image of the first. Echoes from both pulses are collected by the transducer and summed.
• Linear reflectors, such as normal tissue, produce no net signal. However, nonlinear reflectors, such as microbubbles, produce echoes that are asymmetric and do not sum to zero. The result is that echoes from bubbles are detected preferentially using this method, improving image contrast between tissue and microbubbles.
TARGETED CEUS• Microbubbles are attached with ligands that bind certain
molecular markers that are expressed by the area of imaging interest are then injected systemically in a small bolus.
• Microbubbles theoretically travel through the circulatory system, eventually finding their respective targets and binding specifically.
• The ultrasound system converts the strong echogenicity into a contrast-enhanced image of the area of interest, revealing the location of the bound microbubbles.
POTENTIAL APPLICATIONS
• Inflammation: Contrast agents may be designed to bind to certain proteins that become expressed in inflammatory diseases such as Crohn's disease and atherosclerosis.
• Thrombosis and thrombolysis: Activated platelets are major components of blood clots (thrombi). Microbubbles can be conjugated with a ligand specific for activated glycoprotein IIb/IIIa (GPIIb/IIIa), which is the most abundant platelet surface receptor. The microbubbles will specifically bind to activated platelets and allow real-time molecular imaging of thrombosis, such as in myocardial infarction.
• Can also be used to image malignant tissues, as a way to deliver genes and drugs to the tissues.
UNTARGETED CEUS
Benefits:
Organ Edge Delineation: Microbubbles can enhance the contrast at the interface between the tissue and blood.
Blood Volume and Perfusion: contrast-enhanced ultrasound holds the promise for (1) evaluating the degree of blood perfusion in an organ or area of interest and (2) evaluating the blood volume in an organ or area of interest.
Lesion Characterization: contrast-enhanced ultrasound plays a role in the differentiation between benign and malignant focal liver lesions.
APPROVED CONTRAST AGENTS IN INDIA
• SonoVue® - (sulphur hexafluoride microbubbles) Left ventricular opacification / endocardial border definition, breast, liver, portal vein, extracranial carotid, peripheral ateries (macrovascular and microvascular).
• Definity® - (Perflutren lipid microsphere) Left ventricular opacification / endocardial border definition, liver, kidney.
SAFETY PROFILE
• Piscaglia and Bolondi, in a retrospective review of European experience with the use of microbubble contrast agents in more than 23,000 patients, reported adverse effects (AE) in 29 cases, of which only two were graded as serious; the rest, 27, were nonserious (23 mild, three moderate and one severe). The overall reporting rate of serious AE was 0.0086%. Overall, only four AEs required treatment (two serious, two nonserious including one moderate and one severe AEs).
CHARACTERIZATION OF LIVER LESIONS.
In a study by Fuminori et all, the overall rate of correct diagnosis of lesions by the blinded reviewers significantly improved from 68.4% for unenhanced ultrasound to 88.9% for CEUS. In addition, the overall rate of correct diagnosis with CEUS was significantly higher than that with CT (80.5%). In classification of the lesions into the five types, the rates of correct diagnosis of HCC, metastasis, and hemangioma were significantly higher for CEUS than for unenhanced ultrasound. In particular, all 17 cases of hemangioma were correctly diagnosed with CEUS (100%). The performance of CEUS in the correct diagnosis of metastasis was superior to that of CT.
In terms of correct classification of lesions as malignant or benign, the overall accuracy and sensitivity significantly improved from 86.3% and 89.0% for unenhanced ultrasound to 97.4% and 98.8% for CEUS
HEMANGIOMA
HCC
LIVER METASTASIS
SOME OF THE CURRENT USES OF CEUS.
• Contrast enhanced voiding urosonography.
• Monitoring of tumor response to anti-angiogenic therapy.
• Monitoring of local ablative therapy in HCC and metastasis.
• Characterization of kidney lesions.
• Charecterization of liver lesions.
• To characterize pancreatic lesions.
• Blunt abdominal trauma.
• Transcranial vascular imaging.
• Assessment of atherosclerotic plaques.
REFERENCES• US Artifacts. Myra K. Feldman, Sanjeev Katyal, Margaret
S. Blackwood. Radiographics, 2009.
• Sonographic Artifacts and Their Origins. Kathleen A. Scanlan. AJR Am J Roentgenol. 1991 Jun;156(6):1267-72.
• Color Comet-Tail Artifact: Clinical Applications. Hisham Tchelepi, Philip W. Ralls. AJR, 2009.
• Contrast-Enhanced Ultrasound: What Is the Evidence and What Are the Obstacles?. Stephanie R. Wilson, Lennard D. Greenbaum. AJR, DOI:10.2214/AJR.09.2553.
